Cardiac surgery, cardiopulmonary bypass, and preoperative renal dysfunction
Santiago, Chile
* E-mail: glema{at}med.puc.cl
Editor—We read with interest the paper by Loef and colleagues.1 This complex study adds new information to the subject and the authors should be congratulated for their work. We wish to add some comments and ideas to the discussion. Our group published, in 1998, a study of elective coronary patients with abnormal preoperative renal function undergoing surgery with cardiopulmonary bypass (CPB),2 using ‘gold standard’ techniques to measure glomerular filtration rate (GFR) and effective renal plasma flow (ERPF). Further studies by our group have been carried out in children and patients with valvular disease undergoing surgery with CPB, with similar methodology. Our study, however, was intended to focus only in the preoperative period. It is difficult to study cardiac surgery patients after operation, due to the many variables involved, such as changes in haemodynamic variables, use of vasoactive drugs, changes in haematocrit, and bleeding. Thus, the results presented1 are speculative. It is interesting that GFR remains within normal values in patients with preoperative renal dysfunction during both studies,1 2 suggesting that modern techniques for CPB management are able to protect renal function. It also refutes earlier work, suggesting that CPB reduced the GFR in all patients. ERPF increased during CPB in all our earlier studies in coronary patients. In recent studies on valvular and paediatric patients, ERPF remains within baseline values throughout the study. This difference could be related to new strategies during CPB: higher perfusion pressures, vasopressor drugs, and higher haematocrit. We disagree with the authors regarding the explanation for changes found in filtration fraction (FF). We showed in our work that during CPB (hypothermia), FF decreased. FF has been used as an index of vasodilatation or vasoconstriction of the efferent arteriole. The reduction of FF most likely represents a vasodilatation state, due to hypothermia, low viscosity, or the release of vasoactive endothelial factors. A high FF in the postoperative period could represent vasconstriction with unknown consequences for renal function, thus the significance of this finding is unclear. CPB has been blamed for many years for renal dysfunction in cardiac patients; however, in most of the recent trials, CPB has not been an independent risk factor for perioperative renal failure.3 Interestingly, studies with off-pump surgery have shown the same incidence of renal dysfunction in patients with preoperative abnormal renal function.4 We agree with the authors that hypothermia seems to be the best protective intervention in these patients, mainly due to reduced metabolism. Drugs that increase blood flow non-specifically have not shown benefit so far.
Grant FONDECYT 1030645–2003.
Groningen, The Netherlands
* E-mail: b.loef{at}hccnet.nl
Editor—We thank Dr Lema and colleagues for their comments and ideas on the subject discussed. Their remarks concern both the absence of change in GFR and the mechanism of change in FF. With respect to GFR, unfortunately, only a limited number of small studies evaluating the effect of cardiac surgery with CPB on renal function using gold standard techniques have been reported. However, recently Witczak and colleagues5 studied the effect of nifedipine infusion on GFR in patients (n=20) with impaired renal function undergoing CPB surgery. GFR was measured as the plasma clearance of 51chromium-ethylene-diamine-tetraacetic acid before operation and 48 h after operation and creatinine clearance was measured before operation and 0–4, 20–24, and 44–48 h after operation. The authors found no statistically significant change in the GFR or in creatinine clearance over time within, or between, groups. This study supports our observation that modern techniques for CPB management are able to protect renal function in patients with preoperative renal dysfunction.1 2 With respect to FF, we agree that interpretation of our data as to the cause of the increased FF after CPB remains speculative. In patients, with normal renal function (plasma creatinine <1.5 mg dl–1), undergoing coronary surgery, Lema and colleagues6 found an abnormally elevated FF before operation, and a significant decrease during bypass, which returned to abnormally elevated baseline values 1 h after operation FF. The increased FF we observed on day 7 after operation may thus represent an extension of the suggested vasoconstrictive state, with unknown consequences to renal function. However, it may also represent a structural remodelling of the vasculature, particularly in our patients with modest impairment of renal function. Until now, renal function studies in cardiac surgical patients are limited to the hospital period and no long-term follow-up is available. Renal function measurements in cohorts of cardiac surgical patients should extend the hospital period to provide insight into the mechanism of changes in renal parameters, and the possible bearings on the long-term effects of cardiac surgery with CPB on renal function.
References
1 Loef BG, Henning RH, Navis G, et al. Changes in glomerular filtration rate after cardiac surgery with cardiopulmonary bypass in patients with mild preoperative renal dysfunction. Br J Anaesth (2008) 100:759–64.
2 Lema G, Urzua J, Jalil R, et al. Renal protection in patients undergoing cardiopulmonary bypass with preoperative abnormal renal function. Anesth Analg (1998) 86:3–8.[Abstract]
3 Brown JR, Cochran RP, Leavitt BJ, et al. Multivariable prediction of renal insufficiency developing after cardiac surgery. Circulation (2007) 116:I39–43.
4 Di Mauro M, Gagliardi M, Iaco A, et al. Does off-pump coronary surgery reduce postoperative acute renal failure? The importance of preoperative renal function. Ann Thorac Surg (2007) 84:1496–503.
5 Witczak BJ, Hartmann A, Geiran OR, Bugge JF. Renal function after cardiopulmonary bypass surgery in patients with impaired renal function. A randomized study of the effect of nifedipine. Eur J Anaesthesiol (2008) 25:319–25.[Web of Science][Medline]
6 Lema G, Meneses G, Urzua J, et al. Effects of extracorporeal circulation on renal function in coronary surgical patients. Anesth Analg (1995) 81:446–51.[Abstract]
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